(a) Field of the Invention
This invention relates to an aqueous hexaantimony tridecaoxide (Sb.sub.6 O.sub.13) sol having fine particle diameters and excellent stability.
(b) Description of the Prior Art
Since antimony oxide has an ability for making fabrics, fibers, plastic materials, etc. flame retardant, it has been used together with flame retardants such as organic chlorides, or other halogen-containing substances, as a flame retarding synergistic agent to improve the flame retarding effect. However, since all of the conventional antimony oxides are composed of large particles having particle diameters of the size of pigments, there was a defect of worsening the touch, transparency and other physical properties of the product in which such an antimony oxide was used. In order to obviate such a defect, in recent years there have been studied methods for obtaining antimony oxide in fine particle form. Examples of known methods are: a method of producing a colloidal sol of antimony pentoxide (Sb.sub.2 O.sub.5) having particle diameters of from 2 to 100 m.mu., wherein antimony trioxide (Sb.sub.2 O.sub.3) is reacted with potassium hydroxide and hydrogen peroxide (H.sub.2 O.sub.2) in the ratio of about 1:2.1:2 mol to form potassium antimonate, followed by deionization to form said colloidal sol (refer to Japanese Patent Publication No. 11848/1982), and a method of producing aqueous antimony pentoxide colloidal sol, wherein a mixture of antimony trioxide, hydrogen peroxide and water is subjected to reflux for one hour and then the mixture is cooled to form said colloidal sol (refer to Japanese Patent Kokai (Laid-open) No. 123997/1977).
On the other hand, it has been known from old times that the above-mentioned Sb.sub.2 O.sub.5, when heated, decomposes to liberate oxygen, and at temperatures between 780.degree. and 920.degree. C., it transforms into hexaantimony tridecaoxide (Sb.sub.6 O.sub.13), and at higher temperatures, antimony tetraoxide (Sb.sub.2 O.sub.4) is generated (refer to Muki Kagaku Zensho IV-4, Maruzen Co. Ltd., published on May 25, 1954, pp 168-170).
Since the production of Sb.sub.2 O.sub.5 colloidal sol by the known former method (Japanese Patent Publication No. 11848/1982) uses a large quantity of potassium hydroxide, potassium hydroxide remains as an impurity in the reaction product. Therefore, to remove this impurity, this method necessitates a deionization step. Furthermore, in this known production method, when the reaction is carried out at a high concentration, the diameters of the resulting particles become large, so that it is necessary to keep the Sb.sub.2 O.sub.5 in the colloidal sol at a low concentration. Moreover, in order to obtain a high concentration product having sufficient commercial value, an additional evaporation step is required. Thus the method comprises a large number of troublesome process steps. In the latter method (Japanese Patent Kokai (Laid-open) No. 123994/1977), no alkali substance is added and therefore there is no problem of impurity removing operation. However, the reaction speed is very slow, so that it is necessary to carry out the reaction at a high temperature near the boil. Thus there may be a danger of bumping during the reaction. Moreover, once the reaction is initiated, the reaction system is rapidly brought to a high temperature by the reaction heat. This makes the temperature control difficult. Therefore, not only it is impossible to enlarge the production scale, but also high costs of installation and energy are inevitable. Further, Sb.sub.2 O.sub.5 thermally decomposes at relatively low temperatures as previously mentioned, so that its thermal stability is not sufficient. When it is used for melt molding of plastics, bubbles formed by the liberation of oxygen lower the physical properties of the molded product and also worsen the appearance. In addition, the Sb.sub.2 O.sub.5 colloidal sol, when used in mixture with various latexes, it aggregates by the inorganic salt contained in the latex. That is to say the colloidal sol is not chemically stable.
On the other hand, Sb.sub.6 O.sub.13 is produced as an intermediate oxide upon producing Sb.sub.2 O.sub.4 by heating Sb.sub.2 O.sub.5, as mentioned in the above literature. Even if an aqueous fine particle Sb.sub.2 O.sub.5 sol is used as the starting material, the primary particles aggregate and unite to form large particles in the production process, especially in the drying step. Even if the aggregated particles are ground, only large diameter particles of the size of pigments are obtained. Accordingly, an aqueous sol containing Sb.sub.6 O.sub.13 of fine particle diameters has not been known.
We have previously invented, as an improved method of producing the above-mentioned known Sb.sub.2 O.sub.5 colloidal sol, a method of producing colloidal antimony oxide, wherein Sb.sub.2 O.sub.3, hydrogen peroxide and an inorganic alkali substance are reacted in mixture in the mol ratio of 1:1.25 or more: 0.015-0.3 (refer to Japanese Patent Kokai No. 23291/1984 and Japanese Patent Kokai No. 137828/1985). As a result of subsequent research, it has been found that, in the above-mentioned invention, the colloidal antimony oxide obtained by limiting the mol ratio of hydrogen peroxide to a particular range and by selecting a particular inorganic alkali substance, is an aqueous Sb.sub.6 O.sub.13 sol, and that this aqueous sol has excellent chemical stability.